The present invention relates to a method for fabricating an image sensor, and more particularly, to a method for fabricating a complementary metal-oxide semiconductor (CMOS) image sensor.
A typical image sensor is a device for capturing image information using the photo-reaction property of a semiconductive material. The typical image sensor changes different electrical values generated in each pixel into certain levels allowing signal treatments, wherein the pixel senses subjects with different light intensity and wavelengths.
Such image sensors include charge coupled device (CCD) image sensors and CMOS image sensors. The image sensors use a photodiode as a light receiving device. The photodiode absorbs the light imaging an external image, and collects and stores photocharges.
FIG. 1 illustrates a cross-sectional view showing a photodiode of a typical CMOS image sensor. A deep N− type region 15 is formed in a photodiode region of a substrate 11. The substrate 11 includes a P-type (P++) substrate 11A highly doped with P-type impurities, and a P-type epitaxial layer 11B which is formed over the P-type substrate 11A by lowly doping P-type impurities in-situ and performing an epitaxial growth process. A P0 region 16 is formed over the deep N− type region 15. A gate oxide layer 13 and a gate electrode 14 of a transfer transistor (TX) are formed over the substrate 11, next to the photodiode region. Although reference numerals are omitted, lightly doped drain (LDD) spacers are formed over sidewalls of the gate electrode 14. Reference numeral 12 represents an isolation structure.
When light impinges upon the substrate structure, electron-hole pairs are generated by the light near a PN junction region including the deep N− type region 15 and the P-type epitaxial layer 11B. These carriers move to the transfer transistor (TX) by a supplied bias, generating current. Thus, light energy is converted into current.
The PN junction region including the deep N− type region 15 and the P-type epitaxial layer 11B formed below becomes a photodiode. An uppermost portion of the photodiode is doped with P-type impurities, i.e., the P0 region 16, isolating the photodiode region and a silicon surface. Thus, an inflow of dark current caused by a silicon dangling bond of the silicon surface may be reduced. The P0 region 16 is formed using an ion implantation process.
However, damages, which are generated during various plasma processes usually needed in fabricating a wafer, may exist over an upper portion of the photodiode. Such damages may generate dark current. Also, some of the heavy metals composing photoresist may not be removed during a subsequent photoresist layer removal process, and may remain over a surface of the photodiode. In such a case, the heavy metals may diffuse inwardly during a subsequent thermal process and cause the dark current generation. However, it may be difficult to substantially remove the heavy metal contamination caused by the plasma damages or the photoresist layer removal process when fabricating a semiconductor device.